The present invention relates to an economically advantageous homogeneous cation exchange membrane used for the electrolysis of an alkali metal chloride according to an ion exchange membrane process. More particularly, it relates to a homogeneous cation exchange membrane operable at a low electrolysis voltage and, in addition, at a high current efficiency which comprises, in a cation exchange membrane having a multi-layer structure consisting of two or three layers or more each having different equivalent weight (the weight of dry resin containing 1 equivalent of ion exchange group) and/or different kind of ion exchange group used for the electrolysis of alkali metal chloride according to an ion exchange membrane process, at least one outer layer having a lower water content of both outer layers of said membrane being a roughened surface.
The homogeneous cation exchange membrane mentioned in the present invention refers to a cation exchange membrane prepared only from ion-exchange resins without blending with thermoplastic resins having no ion-exchange group.
Heretofore, it has been proposed to lower the membrane resistance of a heterogeneous ion-exchange membrane prepared from the mixture of an ion-exchange resin and a thermoplastic resin by roughening the surface thereof by brushing treatment of flame treatment to expose thereby the ion-exchange resin to the membrane surface (for instance, see Japanese published unexamined patent application No. 47590/1977).
However, in a homogeneous ion-exchange membrane consisting of only an ion-exchange resin wherein ion-exchange resin itself is exposed to the membrane surface, there is no such decrease in membrane resistance by the application of such a roughening treatment.
Furthermore, in the case of a homogeneous cation exchange membrane, it has been known that, if the membrane surface faced to the cathode side is not smooth, hydrogen gas bubbles generated from the cathode by the electrolysis are attached to the membrane surface whereby electrolysis voltage is increased. Accordingly, for the prevention of this phenomenon, it has been commonly practiced heretofore to incorporate a cation exchange membrane into the electrolytic cell so that the smooth surface of the membrane faces toward the cathode (for instance, see Japanese published unexamined patent application No. 131489/1976).
Generally speaking, it is desirable that a cation exchange membrane to be used in electrolysis of alkali metal chloride by an ion-exchange membrane process has a thickness of 1000 microns or less, preferably 200 microns or less, to decrease the membrane resistance. Insufficient strength of such a thin membrane is reinforced with support fibers. As the method to embed the support fibers into the ion exchange membrane, there are, for instance, illustrated one wherein a thermoplastic ion exchange membrane intermediate prepared by extrusion molding is superposed on support fibers and both of them are thermally pressed (heat-press lamination method), one wherein only one surface of the thermoplastic ion exchange membrane intermediate prepared by extrusion molding is subjected to hydrolysis to be converted to the non-thermoplastic, support fibers are contacted the opposite thermoplastic surface and the side contacted the support fiber is evacuated to embed the support fibers into the membrane while the whole composite is heated (vacuum lamination method, Japanese published examined patent application No. 14670/1977), one wherein the resin in a form of fibril and an ion exchange polymer are preliminarily kneaded and the membrane is made of this kneaded resin (fibril method) and the like. According to the heat-press lamination method and fibril method, both surfaces can be made smooth; and according to the vacuum lamination method, the preliminarily hydrolyzed surface is made smooth. Heretofore, the ion exchange membrane was incorporated into the electrolytic cell so that this smooth surface faced toward the cathode side.
Further, there has been proposed recently a cation exchange membrane having a double layer structure consisting of a lower water content layer and a higher water content layer in order to maintain a high level of current efficiency and to lower the electrolysis voltage in the preparation of an alkali metal hydroxide. For instance, there has been proposed a method wherein a membrane having double layers consisting of a layer having a higher equivalent weight and one having a lower equivalent weight (Japanese published unexamined patent application No. 14184/1976) or a membrane having double layers consisting of a layer having sulfonic acid groups and one having weak acid groups (Japanese published unexamined patent application No. 120999/1977) is utilized for the electrolysis of sodium chloride. In such a case, it is inevitable to incorporate the membrane into the electrolytic cell so that the lower water content layer, namely a layer having a higher equivalent weight or one having weak acid groups faces toward the cathode side. When the support fibers are embedded into the cation exchange membrane having such a double layer structure, the procedure was carried out in such a manner that the support fibers were embedded from the side of the higher water content layer and at least the surface of the lower water content layer was made smooth.
However, the present inventors have found as the result of detailed observation of electrolysis that, surprisingly, the amount of hydrogen gas bubbles generated from the cathode and attached to the membrane surface is the least when the homogeneous cation exchange membrane is moderately roughened on its cathode side, whereby the electrolysis voltage is the lowest.
That is, the object of the present invention is, in the case of the homogeneous cation exchange membrane having a double layer structure consisting of layers having different equivalent weights and/or kinds of ion exchange group, to further lower the electrolysis voltage while maintaining the current efficiency at a high level. Such an object can be effectively attained by the present invention wherein the surface of the lower water content layer of the cation exchange membrane is roughened.